1. Field of the Invention
This invention is directed to a shoot-through resistant DC/DC converter. The present invention avoids destructive shoot-through current when two non-diagonal switches of a bridge accidentally close at the same time. A fly-back inductor is provided with two primary windings, one for current in each direction. The fly-back inductor is located inside the switching bridge along with a transformer.
2. Description of the Related Art
Prior art shoot-through resistant DC/DC power converters available on the market can avoid the problem of destructive shoot-through current when two non-diagonal switches accidentally close. However, these converters have problems associated therewith. For example, a bridge variation of a Weinberg converter employs a fly-back inductor outside of the switching bridge. Fly-back refers to a coupled inductor as a secondary output where the current is kept flowing through the inductor by transferring the current from one winding to the other. When switches of the diagonal bridge are closed, energy is stored in the inductor and a transformer acts like a forward converter. When the switches are open, the inductor flies back and transfers the current to the output. As the inductor flies back, the voltage on the primary winding goes to N.sub.x (V.sub.0 +the diode voltage drop). Since one terminal is tied to the input, the other terminal reaches a voltage considerably higher than the line voltage and the bridge sees the higher voltage. In addition, since the primaries of the transformer and inductor are not connected in series after the switches are opened, there is nothing to force the current in each magnetic element to zero at the same time. Instead, the leakage inductance of the transformer and inductor are absorbed by stray and snubber capacitances. If the primary current of the transformer goes to zero before the primary current of the inductor goes to zero, then the transformer output current will go to zero before the inductor output current has reached its peak value. This creates a short "hole" in the output current and is the cause of excessive output noise in a Weinberg converter. The "hole" occurs because the primaries of the inductor and the transformer are not connected in series after the switches are open.
The Weinberg converter applies a voltage much higher than the line voltage to the switching bridge. This makes it inappropriate for circuits that have high input voltages. Further, this creates excessive noise on the output due to "holes" in the output current during switching, especially since the inductor and transformer are not constrained to act in tandem.
A Calkin-Hamilton power converter requires an inductor on the input side. The inductor on the input side prevents a destructive shoot-through current from developing when two of the non-diagonal switches accidentally close at the same time (i.e., "shoot-through" does not occur because a direct short of the lines through the switches is prevented). That is, the inductor acts as a limiting impedance so that the current ramps up and does not immediately shoot toward infinity and the switches do not burn up. There is a different problem, however, in that the current of the inductor must flow continuously. Thus, instead of using pulse width modulation (PWM), the switches must run at a mutual 50 percent duty cycle, i.e., one diagonal pair of switches is closed and then the other diagonal pair of switches is closed, so that the current always has a flow path. A diode (preferably a high voltage, free-wheeling diode) provides the inductor with the necessary complete current path so that the inductor current can flow continuously. To control this device an additional switch is added to provide pulse width modulation. The additional switch and high voltage free-wheeling diode in the Calkin-Hamilton converter adds losses to the circuit as well as makes the circuit more complex.
Because of the problem of shoot-through currents, a need for DC/DC power converter which has a limiting impedance in series with a primary winding, but does not apply a voltage higher than the input line voltage to the switches, has arisen.